P. haemolytica as a pathogen causes serious economic damage to the animal farming industry. Vaccines which have been developed in an effort to control the disease have met with variable but limited success. Because the disease is caused in significant part by the animals' own reaction to P. haemolytica infection, inappropriately designed vaccines may actually worsen the clinical condition of infected vaccinates. Thus, there is a continuing need in the art for safe and effective vaccines which can reduce the morbidity and/or mortality of ruminants due to P. haemolytica. 